The present invention relates to shoe seals for sealing the space between an internal floating roof and an inner tank wall of a storage tank.
Bulk fluids such as petroleum and fuel products are usually stored in large cylindrical tanks. These tanks are commonly designed with internal floating roofs or covers to minimize product losses to the atmosphere. A critical part of the internal floating roof is the sealing mechanism that is installed in the annular space (rim space), between the internal floating roof and the inner wall of the storage tank. This sealing mechanism is designed to allow the internal roof to float on the stored product, to maintain a vapor seal, to move easily within the tank as the product level rises and falls, and to keep the floating roof centered within the storage tank.
It is known to use floating roofs made of steel. Steel floating roofs are structurally strong and are designed to have a large amount of buoyancy. Therefore, when designing sealing mechanisms for steel floating roofs, the forces exerted by the sealing mechanism on the steel roof and the weight of the sealing mechanism have not been problems, because of the relatively large strength and buoyancy of steel roofs.
However, there are several disadvantages with steel roofs. Compared to lightweight internal floating roofs, such as those made from aluminum, steel roofs are expensive to install and maintain. The large weight of steel roofs requires that, for installation, a hole be cut in the side of an existing storage tank for moving the heavy steel roof materials into the interior of the tank. After the steel roof is installed and the hole is patched, the tank must then be hydrostatically tested. A major problem with hydrostatic testing is disposal of the water used in hydrostatically testing the tank. In the face of present strict environmental regulations, it is often very expensive to dispose of the test water which is contaminated with a small percentage of a stored product.
Therefore, it has been found more economical to use lightweight floating roofs, such as aluminum floating roofs. The materials used in aluminum floating roofs are small and lightweight enough to be transported to the interior of the storage tank through an existing manhole in the tank shell, rather than by cutting a hole in the side of the tank. Because of their light weight, aluminum floating roofs require less buoyancy. The cost of labor and materials to install an aluminum internal floating roof is less than that for a steel floating roof, and the maintenance cost for an aluminum floating roof is lower than that for a steel roof. Aluminum floating roofs typically float on pontoons secured to their underside and have the advantages of simple construction and low cost.
A major disadvantage of lightweight roofs is that they cannot support much weight because of the nature of their construction. The comparatively weak strength and weight-bearing capacity of lightweight roofs limits the type of seals which can be used with such lightweight roofs to those which are of relatively simple construction, and particularly those which are light in weight.
In the past, a typical seal for such a lightweight roof would consist of a single flexible wiper blade mounted on the outer rim of the roof and extending into contact with the inner tank wall. The wiper blades were made from a variety of materials, depending upon the product being stored. The single flexible wiper blades are of limited effectiveness in providing a seal and, furthermore, tend to deteriorate rather rapidly, therefore requiring a frequent inspection cycle. In an effort to provide more complete and effective sealing of lightweight floating roofs, various secondary seals have been tried. The presence of such secondary seals enhances the sealing action, but usually at the expense of reduced storage capacity, added weight and complexity. A further problem with the wiper blade type of seal is that no elastomeric material has been found which is resistant to attack from all the various products which are stored in the storage tanks. Therefore, depending on the product stored, the wiper blade type seal must be changed to a wiper blade made from a material which is resistant to attack from the product being stored.
Shoe-type seals typically provide adequate sealing action without the need for a secondary seal. Shoe-type seals can be made of stainless steel, and therefore can be used with any type of product in the storage tank. Shoe seals are heavier than wiper blade type seals and exert more force on the internal floating roof which supports them. Floating roofs made of steel are adequate to support shoe-type seals because of the steel roof's strength and large buoyancy. However, when installing conventional type shoe seals on lightweight internal floating roofs, problems arise due to the inability of the lightweight roof to support the weight of the shoe seal and to absorb the forces generated by the sealing action of the shoe seal.
U.S. Pat. No. 5,078,293 discloses a shoe seal for a lightweight floating roof. The '293 device includes hollow floats which are submerged in the product stored in the tank and provide an upward force on a lever bracket which is attached to the shoe sections. The upward force generated by the buoyant member changes in proportion to the density of the product stored. It is evident that the '293 device does not address the important geometrical property of the maximum angle that the shoe bracket makes with the vertical. It is also evident that the friction on the tank shell is proportional to the pressure applied by the shoe and the pressure applied by the shoe is proportional to the upward force from the buoyant member. Therefore, if the product stored in the tank changes such that a significant increase in product density occurs, the friction force between the shoe and the tank shell could increase dramatically, and, combined with unsuitable bracket geometry, could bind and overcome the flotation available. In such a case, the effectiveness of the entire floating roof would be compromised.